Photographic emulsions and elements containing dextran



. 10,000 so that the dextran constitutes to 50% duo-silver halide photographic emulsion.

United rates Patent. Ofiice 3,063,838 Patented Nov. 13, 1962 3,063,838 PHOTOGRAPIHC EMULSIGNS AND ELEMENTS CONTAINING DEXTRAN Andrew Bradshaw Jennings, Wilmington, Deb, asslgnor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Nov. 21, 1958, Ser. No. 776,660

Claims. (Cl. 96-67) v emulsions.

It is known to add certain chemical compounds, e.g., heavy metal and noble metal salts to improve the sensitometric characteristics of photographic gelatino-silver halide emulsions. There is, however, at the present time, a practical limit to the advantages which can be obtained by the addition of such compounds without accompanying deleterious eifects such as uncontrollable fog and instability on tropical aging.

'It has been proposed to add to, or replace all or part of the gelatin in a gelatin silver halide system with various polymeric colloid materials for various reasons including attempts to overcome the well-known disadvantages of gelatin. However, none of these proposals have overcome these disadvantages in a satisfactory manner, and gelatin is still used in most commercial photographic film. In general, when all of the gelatin is replaced by the synthetic polymers suggested by the prior art, the outstanding photographic and colloidchemical properties of gelatin are lost. In addition, it is difficult to determine theoptimum conditions for the addition of coating aids, coating and drying, etc. to form a photographic element possessing satisfactory hardness and stability. This is particularly true when the exposed element contains the usual photographic hardeners and is placed in aqueous photographic processing solutions.

Evva, Zeitschrift fiir Wissensch aftliche Photographie, Photophysik und Photochemie 52, 1-24 (1957), describes silver halide emulsions using dextran, a polyglucose linked in the 1-6 fashion. The use of dextran alone as a binding agent for silver halide grains, however, has various drawbacks including the disadvantages mentioned above. For instance, it is difiicult or impossible to set by chilling, an .emulsion containing dextran as the sole colloid binding agent in the manner commonly used for gelatin emulsions. A further disadvantage of dextran is its inferior protective colloid action for silver halide grains.

-It has now been found, in accordance with this invention, thatgelatino-silver halide emulsions of increased covering power and enhanced properties can be made by using an amount of the water-soluble polyglucose, dextran, having an average molecular weight of at least and prefby weight of the total solids in a gelat- The amount of dextran present is based on finished emulsion ready for coating onto the hydrophobic film, paper, metal foil or plate or other support.

The dextran can be incorporated with the gelatin silver halide at any stage after precipitation of the silver halide grains in the gelatin but is preferably admixed after the digestion step.

erably 7 to -In general, the finished emulsion will contain on a dry basis 5 to dextran, 20 to gelatin, and 40 to silver halide, all by weight. The emulsions may, of course, contain small amounts of conventional adjuvants. The final emulsions have markedly increased covering power (e.g., 10 to 50% greater) over all-gelatin emulsions, which is quite surprising. To be more specific, with respect to covering power for the same quantity of silver halide, large increases in maximum density and contrast can be obtained in the developed image. Covering power can be expressed as the numerical result of dividing optical density by the grams of silver per square decimeter in the developed image layer. The covering power will, of course, vary with the amount and the molecular weight of the dextran used.

The invention is especially suited to dextran/gelatinosilver halide emulsions for radiological films, particularly those for medical diagnostic work. However, the invention is by no means limited to any particular type of dextran/ gelatin emulsion and the dextran may be utilized in any gelatino-silver halide system to improve its eificiency. The invention is also very useful in the socalled graphic arts films, i.e., lithographic films. In all cases when dextran is present in the amounts set forth above, a higher density is obtained from a given amount of metallic silver in the developed photographic layer. Although the efrect is more noticeable in large grain emulsions, it is also ettective in small-grain emulsions.

The processes of the invention are quite simple and consist of merely admixing with the molten or liquefied emulsion after the digestion step an aqueous solution 'of a dextran having a relatively high molecular weight. As an exemplary procedure, a gelatin-silver iodobromide emulsion useful for X-ray [films is prepared by conventional methods. 'Ihe silver halides are precipitated in gelatin and the resulting dispersion or emulsion ripened. The emulsion is then washed, either after chilling and noodlin-g, or after coagulation by decanting the supernatant liquid. The emulsion is then redispersed and then it is digested to bring it to maximum speed. Following digestion, dextran (usually in an aqueous solution) having an average molecular weight of at least 10,000 is added .to the molten emulsion in an amount sufiicient to provide the desiredratio of the dextran to total solids. The usual final additions, e.g., of hardener, antifogging agents, sensitizing agent and spreading agent, are made and the'emulsion is coated on a suitable support and dried in theusual manner. To test the effectiveness of the dextran, the element is exposed in a sensitometer according to a procedure based on the American Standard Method for the Sensitometry of Medical -ray Flms PH 2.9-1956.

The sensitometer used in the .following examples Was equipped with a neutral density V5 step Wedge. The density of a selected step, as set forth in the tables, was

measured and the quantity of metallic silver per square decimeter was determined by analysis. The density divided by the quantity of silver in grams per square decimeter was taken as an expression of covering power.

The invention will now be further illustrated by, but is not intended to be limited to, the following examples. The quantities of dextran are given as a percentage of ,the weight of total solids in the emulsion. In all cases the ratio of gelatin to silver halide is approximately 1.24 to 1.

Example 1 A high-speed gelatino-silver iodobromide emulsion was made in the usual manner and digested with an organic sulfur compound and gold chloride. It contained approximately 1.6 mol percent of silver iodide and 98.4 mol percent of silver bromide. The emulsion was divided into four parts and to each of three portions there was oper of the composition:

Grams .p-N-methylarninop'henol hydrosulfate 3 Anhydrous sodium sulfite 50 .Hydroquinone 9 Anhydrous sodium carbonate 50 Potassium bromide 4.5

Water to :make 1000.0 ml.

Following development, the films were fixed, washed and dried in the conventional manner. The results of the sensitornetric tests and measurements of covering power, are shown'in the following'table:

Coverin Percent Coating Maximum Power at Dexticn Weight Density Density Example 2 An emulsion was made and treated in the'same manner min-Example 1 except that a 10% aqueous solution of dextran having a molecular weight of from 15,000 to 20,000 was added in place of the dextran of that example. The results are shown in the following table:

Example 1 was repeated except that covering power was determined at maximum density instead of the constant density :of 1.5. The results were as follows:

1 Covering Percent Coating Maximum, Power at Dextran Weight Density Maximum Density Example 4 Example 2 was repeated with covering powers being determined at maximum density. The results are shown in the following table:

' Covering Percent Coating Maximum Power at Dextran Weight Density Maximum Density .0 03. 9 l. 65 30 V 7 93. 2 l. 63 32 111 92. 8 1. 75 '34 14 92. 3 1. 99 38 An emulsion was made as described in Example 1 and divided into four portions, one of which was used as a control. To the remaining three portions there was added enough of a 10% aqueous solution of crude, unfractionated dextran to give the percentages based on the weight of total solids in the emulsion as shown in the following table. Exposure and processing procedures were the same as those in Example 1. The results are shown in the following table:

Covering Percent Coating Maximum Power at,

- Dextran Weight Density Maxi1nu1n Density 1 (Control) -l 0 '93. 9 '1. 05 '30 2 '7 92. 0 l. 83 36 11 99. 4 2. 04 38 14 96. 1 2. 19 42 It has been found that the increased covering power of photographic emulsion layers is not limited to adding a dextran to the gelatino-silver halide emulsion layer. It can be attained by incorporating the dextran in a gelatin composition to be coated next to a gelatino-silver halide emulsion layer, e.g., in a gelatin sublayer, a separator or light-filtering layer or in an antiabrasion layer. The amount of dextran so used can be used on the total gelatin in the contiguous layers. A layer essentially composed of a dextran, e.g., 75 to 100% dextran, can be used in a layer contiguous with a gelatino-silver halide emulsion layer. This is illustrated by the following example.

Example 6 A 6% aqueous gelatin solution was made and divided into 6 portions. To each portion enough dextran was added to give the percentages based on total solids shown in the table below. The table also shows the coating weights of the undercoatings and of the overcoated emulsion which is the same as that described in Example '1 used as the control.

Undercoat Emulsion Covering Coating; Maximum Fog Power at; Percent Coating rngJdmF, Density Density dextran Weight, AgBr 1.8 1 mgJdm.

0 60 103. 0 1.81 i 14 33 0 80 100. 7 1. 77 08 v34 25 60 100. 2 l. 98 ll' 42 25 80 102. 9 2.06 .15 42 90. 7. 2. 25 .13 15 40 102.9 2. 32 l5 46 Example 7 Example VI was repeated except that covering power was determined at a density of 1.3 and one sample contained at higher quantity of 'dextran.

i Covering Percent Coating Maximum Power at Dextran Weight Density Denzslity i l.

As will be apparent from the above description, the invention is not limited to the specific quantities or molecular weights of dextran as shown in the examples. Dextran of higher molecular weights, e.g., up to about 100,000 and more as well as dextrans having a molecular weight as low as about 10,000 may be used. Dextran is a linear glucose polymer in which the units of one chain are in chemical union with members of adjacent chains to form a network structure. The main chain of the polymer is made up of glucopyranose units in a 1,6-linkage with the side chain being a 1,4-linkage.

The invention is not limited to photographic gelatinosilver halide emulsions of the silver iodobromide type. The invention may be applied to other gelatino-silver halide emulsions, e.g., gelatino-silver bromochloride emulsions of the lithographic type. The invention is particularly efficacious in photographic emulsions whose average grain size is relatively large. The emulsions may contain any of the Well-known optical sensitizing dyes as well as non-optical sensitizers such as sulfur sensitizers containing labile sulfur, e.g., allyl isothiocyanate, allyl diethyl thiourea, phenyl isot 'ocyanate and sodium thiosulfate; the polyoxyalkylene ethers disclosed in Blake et al., U.S. Patent 2,400,532 and the polyglycols disclosed in Blake et a1. U.S. Patent 2,423,549. Other non-optical sensitizers such as amines as taught by Staud et a1. U.S. Patent 1,925,508 and metal salts as taught by Baldsiefen U.S. Patent 2,540,085 and Baldsiefen et al., U.S. Patent 2,540,086 may also be used. Antifoggants, e.g., benzotriazole and triazaindenes can be used as well as the usual hardeners, i.e., chrome alum, formaldehyde, etc. The emulsion may be coated on any suitable support such as paper or films composed of cellulosic esters, e.g., cellulose tn'acetate, cellulose acetate/butyrate; super polymers, e.g., polyvinyl chloride co vinyl acetate); polyvinyl acetals, e.g., formal and acetal; polystyrene; polyamides, e.g., polyhexamethylene adipamide, and polyesters, e.g., polyethylene terephthalate. The vinylidene chloride copolymer-coated oriented polyester films of Alles et al., U.S. Patent 2,627,088 are especially suitable.

An advantage of the invention is that it provides a simple, dependable and effective means for providing gelatino-silver halide photographic emulsions of enhanced covering power. The efliciency of the resulting developed silver is increased, that is, it can provide greater density per quantity of metallic silver which results from development.

Another advantage of the invention is that the addition of dextran to photographic emulsions requires no special technique and can be carried out by the ordinary technician with conventional apparatus. Since dextran can be added from an aqueous solution and it does not provide the problems of solvent recovery, the resulting modified gelatino-silver halide emulsion can be coated and dried in the conventional coating and drying apparatus Which oflfers commercial advantages. Still further advantages will be apparent from the foregoing description of the invention.

What is claimed is:

1. A photographic silver halide emulsion comprising, on a dry weight basis, gelatin and 40 to 70% light-sensitive silver halide.

2. An emulsion as defined in claim 1 wherein said dextran has an average molecular weight from about 10,000 to about 100,000.

3. An emulsion as defined in claim 1 wherein said silver halide is silver iodobromide.

4. An emulsion as defined in claim 1 embodying a sulfur sensitizer containing labile sulfur.

5. A process which comprises admixing with a digested gelatino-silver halide emulsion containing, by weight, 20 to gelatin and 40 to light-sensitive silver halide, from 5 to 50% by weight of dextran.

6. A process as defined in claim 5 wherein said dextran has an average molecular weight from about 10,000 to about 100,00

7. A process as defined in claim 5 wherein said silver halide is silver iodobromide.

8. A photographic element comprising a support and a gelatino-silver halide emulsion layer comprising, on a dry weight basis, 5%-50% dextran, 20-55% gelatin binder and 4070% light-sensitive silver halide.

9. An element as described in claim 8 wherein said silver halide is silver iodobromide.

10. A photographic element comprising a support, a gelatino-silver halide emulsion layer and a layer contiguous with the silver halide emulsion layer, the said contiguous layer containing to 100% dextran, any remaining constituent of the contiguous layer being gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 525,512 Prestwich Sept. 4, 1894 2,739,891 Knox Mar. 27, 1956 2,850,398 Witt Sept. 2, 1958 2,937,085 Seven May 17, 1960 OTHER REFERENCES Evva: Kolloid Zeitschrift, vol. 135, pages 136l40, March 1954.

Mees: The Theory of the Photographic Process, Revised Ed., Macmillan Co., New York, 1954, pages 87- 90 and 112.

Evva: Zeitschrift fiir wissenschaftliche Photographie, Photophysik und Photochemie, 52, 1957, pages 1-24.

5 to 50% dextran, 20 to 55% 

10. A PHOTOGRAPHIC ELEMENT COMPRISING A SUPPORT, A GELATINO-SILVER HALIDE EMULSION LAYER AND A LAYER CON-TIGUOUS WITH THE SILVER HALIDE EMULSION LAYER, THE SAIDD CONTIGUOUS LAYER CONTAINING 75% TO 100% DEXTRAN, ANYY REMAINING CONSTITUENT OF THE CONTIGUOUS LAYER BEING GELA-TIN. 